This invention relates to an improved process for the preparation of aliphatic polycarbonates by transesterification of dihydric alcohols with glycol carbonate.
It is known to prepare polycarbonates from aliphatic dihydroxyl compounds either by a process of phosgenation in which hydrogen chloride is liberated or bound by bases, such as pyridine or quinoline, or by a process of transesterification with carbonic acid esters of alcohols or phenols, preferably diphenylcarbonate, optionally with the aid of transesterification catalysts.
In either case, it is essential to use phosgene or a mixture of carbon monoxide and chlorine as source of carbonic acid. Technical processes which involve the preparation and handling of phosgene are different and costly on account of the of considerable safety risks involved and the high cost of materials due to corrosion. To this are added ecological problems since either the spent air is contaminated with hydrogen chloride or the effluent water with sodium chloride.
The process according to the present invention, on the other hand, is ecologically harmless since the source of carbonic acid used is the carbon dioxide from the atmosphere, which may easily be converted to glycol carbonate by reaction with ethylene oxide by known methods.
Although a process for the preparation of linear polycarbonates which is based on the reaction of selected aliphatic dihydroxyl compounds with glycol carbonates in the presence of basic transesterification catalysts followed by separation of the resulting glycols in a fractionating column has already been described in German Offenlegungsschrift No. 1,495,299 this process has various disadvantages which considerably restrict its technical usefulness. It is obviously only suitable for the production of cycloaliphatic polycarbonates based on relatively non-volatile dihydroxyl compounds, such as cyclohexane dimethanol and perhydrobisphenol. Even in that case, it is necessary to use a considerable excess of glycol carbonate because ethylene glycol cannot be completely separated even if a long fractionating column is used. Additional complicated operations are therefore required for recovering and purifying the excess ethylene glycol which is carried along.
According to German Offenlegungsschrift No. 1,495,299, the preparation of pure aliphatic polycarbonates (e.g. those based on hexane-1,6-diol) which are much more valuable in the field of polyurethanes, which is the application in which aliphatic polycarbonates are preferably used, gives rise to considerable difficulties. The yields are in these cases unsatisfactory and the molecular weights of the resulting products obtained are either too low or the OH numbers are too high for optimum effects to be obtained when the products are processed to polyurethanes.
There was therefore a technical demand for the development of a transesterification process by which aliphatic polycarbonates which would be optimally useful in the polyurethane field could be obtained and which would also be economical and uncomplicated in operation.